Cancellation of side lobes are often used in radar systems in order to cancel jammer signals picked up in side lobes of a radar antenna. The existing solutions allow jammer signals from several jammer sources to be cancelled. One solution is described in U.S. Pat. No. 4,044,359 “Multiple Intermediate frequency side-lobe canceller”. Side lobes are difficult to avoid when designing antennas and jammer signals can be picked up by these side lobes. As the jammer signals are often powerful they will cause interference with the signal from a target picked up in the main lobe even if the reception sensitivity of the side lobe is much below the reception sensitivity of the main lobe.
The principle of operation of prior art solutions is shown in FIG. 1 with a main antenna 101, an auxiliary array antenna 102 having N antenna elements A1 to AN, a control unit 103 and a cancelling circuit 104 with an output terminal 111. The main antenna has a main lobe 104 and side lobes 105-108. Jammer signals are emitted from jammer sources J1-J3. As can be seen in FIG. 1 jammer signals from J1 are picked up by the side lobe 108 in the main antenna and by the antenna elements in the auxiliary antenna 102. Signals s1-sN from the antenna elements A1 to AN are fed to the control unit 103. The control unit produces error signals e1-eN which are fed to the cancelling circuit 104. The output from the main antenna 101 is fed to the cancelling circuit 104. There is also a feedback signal 112 from the output terminal 111 of the cancelling circuit to the control unit 103. In the control unit the feed back signal will be used to affect amplitude and phase of the input signals s1-sN to produce the error signals e1-eN. When the error signals are subtracted in the cancelling circuit with the undesired signal from side lobes of the main antenna they will cancel the jammer signal so that only the signal representing the main lobe and non affected side lobes will be available at the output terminal 111. Thus the undesired signal/s picked up by the undesired side lobe/s is/are cancelled.
The drawback with the existing solutions today is that only narrow band cancellation of the side lobe is possible. This is a serious problem as radar antennas are often operating over a very wide bandwidth and the control unit is only capable of producing the error signals over a narrow bandwidth resulting in that the side lobe cancellation is only effective in a part of the operating bandwidth of the radar antenna.
Thus there is a need for an improved solution for providing a side lobe cancellation over a wide bandwidth in order to cancel jammer interference over a wide bandwidth.